Adds components/history tests to BUILD.gn

components/history/core/common/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 static_library("common") {
   sources = [
     "thumbnail_score.cc",
     "thumbnail_score.h",
   ]
 
   deps = [
     "//base",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "thumbnail_score_unittest.cc",
+  ]
+  deps = [
+    ":common",
+    "//testing/gtest",
+  ]
+}